pgsql: In SRF example, move oldcontext variable definition into the

Log Message:
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In SRF example, move oldcontext variable definition into the FIRSTCALL
branch, which is how most actual code is actually structured.  Also fix
slight whitespace misalignment.

Modified Files:
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    pgsql/doc/src/sgml:
        xfunc.sgml (r1.141 -> r1.142)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/xfunc.sgml?r1=1.141&r2=1.142)